Vacant cardinal positions are awarded to bishops by the Pope based on their Cardinal appointment score, which relies on factors about the candidate and factors about his liege.

Rulers may invest in a campaign fund, which will increase the chances that the candidate may be chosen.

The cardinals choose the next Pope based on a Papal suitability score, which relies solely on the factors about the candidate and thus rulers cannot directly influence. Pope who received votes from cardinals from a particular ruler's realm will have a massive +100 relations modifier per vote received.

Factors used both for Cardinal appointment and Papal suitability :

Factor

Score

Comment

Opinion

+3.5 x Pope's or Cardinal's opinion of candidate

Secular power

Sum of power of titles

Rank and number of titles held by the candidate:

Bishop: +10 or +20 (?)

Prince-Bishop: +60

Prince-Archbishop: +130

King-Bishop +70

Culture = Italian

+200

Based on the province culture of Rome. Will change if Rome changes culture; will not change if the Pope moves capitals.

Culture group = Latin

+50

Does not stack with Italian culture bonus. Based on the province culture of Rome. Will change if Rome changes culture; will not change if the Pope moves capitals.

If you have the patience, select future cardinals at a young age (6-10) based on their early base stats and traits. Genius and Quick are great because they provide rare boosts to the Learning attribute.

The ideal guardian has high diplomacy, high learning skill, and 4+ good traits. Heavenly virtues are easy to scan for and all are helpful:

Trait

Diplomacy

Learning

Church opinion

Cardinal score

Monthly Piety

Guardian*

Chaste

✗

+1

+10

✓

+0.5

✗

Temperate

✗

✗

+10

✓

✗

✗

Charitable

+3

✗

+10

✓

✗

✗

Diligent

+1

+1

✗

✗

✗

✓

Patient

+1

+1

✗

✗

✗

✗

Kind

+2

✗

✗

✗

✗

✓

Humble

✗

✗

✗

✗

+1

✗

* Generally improves decisions made by guardians.

In time, the ideal educator for a potential future cardinal will be one of your other cardinals or future cardinals—or yourself, for maximum control over the child's traits (unless you are arbitrary/cruel).

It is very helpful for your future cardinal to have Italian culture. If the child is not Italian, see culture conversion by education. Consider converting to Italian yourself. You can always search for Italian characters by right-clicking any Italian character, selecting "go to character relations", and limiting the search to "my culture" [sic]. Alternatively, try to convert Rome to your culture, so it is your culture (rather than Italian) that gets the cardinal appointment score boost.

Just before the age of 16, switch the child to a Mastermind Theologian if their guardian is not one.

When he reaches the age of majority at 16, order him to take his vows. Becoming a monk will remove him from any succession lists, give him a monthly piety boost, and increase everyone's opinion of him by +10. But make sure the interaction says he will remain in your court: you don't want him to join a holy order that isn't your vassal.

If he is the son of a ruler and has high martial skill, try to get him a bishopric quickly, or he may try to join a holy order (costing you piety if you refuse).

If a Crusade is called, have him lead an army in the target kingdom to get the Crusader trait.

The secular power of the bishop is a major factor in cardinal selection. Most candidates are mere baron-level bishops; yours can have an edge. Furthermore, higher-rank clergy have more chances to gain monthly piety from church buildings. Their increased demesne limit lets them hold more churches, and they can have more vassals with churches, both baron-level and count-level.

You can create county-level bishoprics out of Holy Warred lands, or by revoking a county and granting it to a local bishop. You can then grant a duchy to the prince-bishop, making him a prince-archbishop. The technique is largely the same as for creating vassal republics.

A kingdom-level theocracy is huge "secular power". You cannot grant titles higher than duke directly to bishops, but you can help a Prince-Archbishop create a kingdom. He will need two duchies and control over 51% of a de jure kingdom, which you can arrange by granting him titles directly or transferring counts under him. (You can transfer counts even when your realm is over 10% vassal theocracies.) Alternatively, you make a kingdom claimant a bishop, and push his claim.

Keep in mind that you cannot create more theocratic counties if more than 10% of your realm is controlled by theocracies. You must choose between having a large number of count-level direct vassals (preferably in safe, uncreated duchies) or a small number of higher-rank vassals.

If you have lots of money, construct piety-generating buildings in the temples belonging to your cardinals and their direct vassals. You can also build new temple holdings for them, even in counties controlled by feudal rulers—you are not obligated to transfer the temple holding to the local count.

If you have Papal Investiture, nominate the member for the priesthood by asking the Pope himself. If he consents to the nomination then your designated family member will succeed to the next available bishopric within your realm.

Assassinate bishops in your realm to speed up investiture.

Holy warred counties can be converted to theocracies and given to your chosen person at the same time, bypassing the need for investiture (once).

To acquire the "In Rome" modifier, appoint your potential Cardinal as your Court Chaplain and send him to Rome on a mission to improve religious relations.

If all else fails, contribute money to his campaign fund until he is the best candidate. You should contribute at least some money for the "Supported Cardinal Election" opinion boost, which gives +30 for 83 years.

If you're feeling particularly devious, you could assassinate a Cardinal (or Pope) to make room for a new cardinal.